(A U B)' is the complement of the set A U B ... the set of all elements not in
A U B.
A' U B' is the union of set A's complement and set B's complement.
more info ...
Complement Of A Set (with worked solutions)
it is very simple
i will try to explain u by an example
if Z = the set of numbers from 1,2,3.....10
then A = the set of numbers, that is 2,6,10
and B = the set of numbers, that is 1,3,5,9
A' means the set of numbers which is not in A
then A' = 1,3,4,5,7,8,9
like that B' = 2,4,6,7,8,10
then A' U B' is the set of all numbers in A' and B'
that is A' U B' = the set of numbers that is 1,2,3,4,5,6,7,8,9,10.
now u may understood that apostrophe denotes "not in the set"